There are various gases whose detection is desired. Examples include exhaust gases emitted from automobiles, the emission amount of which is regulated, toxic gases discharged from home hot-water heaters under abnormal conditions, and gases used for fuel cells which have been developed in order to overcome recent environmental problems.
For example, there may be mentioned CO2, NOx, and SOx in the exhaust gases emitted from automobiles, CO as a toxic gas discharged from home hot-water heaters, and H2 as a gas discharged from fuel cells. In particular, since gas leakage of CO, H2, and the like may cause serious accidents, a gas sensor which can accurately and rapidly detect a detection target gas has been increasingly demanded as time progressed.
For example, since H2 has a low explosion limit of 4%, a hydrogen gas sensor for fuel cells is required to rapidly detect a small amount of gas. Being used under severe conditions, this type of sensor is also required to have excellent environmental resistance. Furthermore, in order to promote wide spread of fuel cells, inexpensive sensors have also been desired.
Several gas sensors using exothermic heat or endothermic heat generated by a catalytic reaction of a noble metal, such as Pt or Pd, have been proposed. In particular, a gas sensor which detects a detection target gas by converting the heat of an exothermic or an endothermic reaction performed in a catalytic layer into a temperature-dependent resistance value of a thermistor has been disclosed in Patent Document 1.
With reference to FIG. 1, the operation of the gas sensor disclosed in Patent Document 1 will be described.
As shown in FIG. 1, the gas sensor of Patent Document 1 includes on a substrate 1 used as a support member, a thermistor layer 2, electrodes 3a and 3b connected to two ends thereof so as to flow current in the thermistor layer 2 in parallel with a film surface thereof, an insulating layer 4 formed on the thermistor layer 2 and the electrodes 3a and 3b to prevent a short circuit between the electrodes 3a and 3b, and a catalytic layer 5 formed on the insulating layer 4.
The electrodes 3a and 3b are connected to the two ends of the thermistor layer 2 with a predetermined gap therebetween so as to have flow current in the thermistor layer 2 in parallel with the film surface thereof. As a material for the electrodes 3a and 3b, for example, there may be mentioned a metal material, such as aluminum or platinum, which is used for common electrodes.
The insulating layer 4 is a layer which prevents short circuit between the electrodes 3a and 3b, and for example, an insulating resin or ceramic may be used.
The catalytic layer 5 is formed of a material which causes an exothermic or an endothermic reaction upon interaction with a detection target gas, and for example, an oxidation catalytic material which catalyzes oxidation of a detection target gas or a hydrogen storing alloy may be used.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2005-98846